This invention relates to a compression-type semiconductor device in which such semiconductor elements as transistors, thyristors and gate turn-off thyristors are maintained in a compressed state.
Compression-type semiconductor devices having such semiconductor elements as transistors, thyristors and gate turn-off (GTO) thyristors which are maintained in compressed states are widely known as power devices. FIG. 1 shows the general construction of a semiconductor device of this type. Columnar metal stamps 14, 15 composed of a material such as copper having high thermal and electrical conductivity are disposed on both sides of a semiconductor element 11. Metal plate 12 is interposed between semiconductor element 11 and metal stamp 14, and metal plate 13 is interposed between semiconductor element 11 and metal stamp 15. As denoted by the arrows in FIG. 1 the semiconductor element 11 is compressed between metal plates 12, 13 by the metal stamps 14, 15. The thermal expansion coefficient of semiconductor element 11 generally differs from that of metal stamps 14, 15; so that there were metal stamps 14, 15 in direct contact with the semiconductor element 11, the temperature change accompanying the operation of the semiconductor device would result in a bimetal effect causing mechanical stress to be exerted on semiconductor element 11. In order to prevent this bimetal effect, the metal plates 12, 13 are formed of a material having a thermal expansion coefficient intermediate those of the semiconductor element 11 and metal stamps 14, 15. In general, materials such as molybdenum and tungsten which have thermal expansion coefficients close to that of the semiconductor element 11 are used for metal plates 12, 13. Where, for example, semiconductor element 11 is a thyristor having an anode electrode on one surface thereof and a cathode electrode on the opposite surface therefrom, one surface of metal plate 13 is directly bonded by an alloying method to the surface of semiconductor element 11 which contains the thyristor anode electrode. The other surface of plate 13 is bonded to the metal stamp 15 with a solder layer 17. One surface of metal plate 12 is bonded to metal stamp 14 with a solder layer 16, while the other surface thereof is pressed against the surface of semiconductor element 11 which contains cathode electrode.
FIG. 2 illustrates a conventional semiconductor power elements such as a thyristor or diode, having a cathode electrode 21 and a gate electrode 22. As shown, cathode electrode 21 is substantially an electrically integral structure. Therefore, no serious change in electrical properties is brought about if the semiconductor element is compressed somewhat unevenly.
However, in a majority of the semiconductor elements, which have recently attracted attention in this field, such as high power transistors and GTO thyristors, a cathode region 31 (or emitter region) is divided into a plurality of mesa portions, as shown in FIG. 3A. Separate electrodes (not shown) formed on each separate section of the cathode region 31 are pressed into contact with a metal plate disposed within a region 32 denoted by a dashed line. Where the semiconductor element is a GTO thyristor, the divided sections of the cathode region 31 are allowed to operate simultaneously as independent GTO thyristors, thereby performing the gate turn-off function for a large current. Thus, it is absolutely necessary in this instance for the metal plate within region 32 to press uniformly upon each of the electrodes within cathode region 31. Otherwise, unevenness in the current distributed among the individual elements of cathode region 31 or unevenness in turn-off properties can result, due to partial contact or differences in contact resistance between some individual elements of cathode region 31 and the metal plate within region 32.
We have recognized that the compressed state of a semiconductor device of this type basically can be represented by the model of a rigid post (corresponding to the metal plate and metal stamp) pressed against a semi-infinitely elastic body (corresponding to the semiconductor element). Assume, for example, that a rigid post 42 is pressed against a semi-infinitely elastic body 41, as shown in FIG. 4A. In this case, the stress P(x) generated in elastic body 41 in a direction perpendicular to the contact plane is represented by formula (1) below according to the description found at page 45 of "Theory of Elastic Contact" by Garlin and page 1085 of "Handbook on Design of Strengh": ##EQU1## where, q=pressure applied to the rigid post 42,
R=radius of the rigid post 42, and PA1 x=distance from the center of the rigid post 42.
In FIG. 4B which graphically depicts formula (1) it can be seen that the stress generated in the semi-infinitely elastic body 41 gradually increases toward the periphery of rigid post 42, reaching infinity in the region contacting the periphery of the rigid post. This suggests that the stress tends to concentrate in the peripheral portion of the semiconductor element in compression-type semiconductor devices. As a matter of fact, GTO thyristor devices damaged during operation have been examined, and in the majority of devices, a circular impression along the dotted line 32 of FIG. 3A was found. Also, a marked reduction in the maximum operational anode (or controllable) current occured in GTO thyristor devices bearing such circular impressions. It is believed that the uneven stress distribution shown in FIG. 4B gives rise to unevenness in the planar distribution of anode current, bringing about a reduction in the maximum operational anode current mentioned above. In addition, in GTO thyristor devices bearing such circular impressions, peripheral portions of the cathode electrode are forced radially outward, due to thermal fatigue during operation, with the result that the cathode electrode directly contacts the gate electrode, short circuiting the device.
In order to overcome the difficulties described above, it has been suggested that unevenness of stress distribution could be moderated by providing a metal stamp 51 with a recess 51a as shown in FIG. 5. In this case, however, the deformation of a metal plate 52 disposed between the metal stamp 51 and a semiconductor element (not shown) varies with the thickness of the metal plate 52, failing thereby to provide a consistently reliable solution to the problem of uneven stress distribution.
It has also been proposed, as shown in FIG. 6, to cut at an acute angle .theta. the periphery 62a of the side of a metal plate 62 which contacts semiconductor element 61. The cutting angle .theta. which is determined arbitrarily, is customarily 30.degree. or more. In general, the metal plate 62 is about 500 to 1000 .mu.m thick, and the cut portion at the periphery 62a of the metal plate 62 has a height of about 100 to 300 .mu.m. Nevertheless, because the cathode region of the semiconductor element 61 is only about 10 to 30 .mu.m high, as seen in FIG. 6, cutting the periphery 62a of the metal plate 62 at angle .theta. only shifts the periphery of contact between the semiconductor element 61 and metal plate 62 from point P radially inward to point Q, which does not significantly eliminate uneven stress distribution.
Therefore, solving the problem of uneven stress distribution in a compression-type semiconductor element remains a matter of serious concern.